Diurnal variability of mesospheric ozone as measured by the 183-GHz radiometer of the UARS microwave limb sounder (MLS) instrument for the northern midlatitudes in October 1991 and 1992 is compared with theoretical calculations of diurnal amplitudes produced by two photochemical models and with ground-based microwave measurements made from Bordeaux (France, 45 ¿N) in October 1988, 1989, and 1990 and the Table Mountain Facility (California, 35 ¿N) in October 1990. Great care has been taken in comparing all the data sets within the same frame, i.e., interpolating onto the same vertical grid (pressure or altitude), using the same units (concentration or mixing ratio) and degrading the vertical resolution of some data or models (convolution of the vertical profiles with appropriate averaging kernels). MLS diurnal variability generally agrees to within 10% with ground-based and model results at 0.5, 0.2, 0.1, and 0.05 hPa (approximately 55, 60, 65, and 70 km, respectively). Although modeled diurnal changes at 55¿8 km are closer to the ground-based Bordeaux measurements than to the MLS data at 45 ¿N, MLS results are closer to ground-based Table Mountain Facility data at 35 ¿N at 0.42 and 0.22 hPa (~55¿8 and ~60¿8 km, respectively) than to models. At 0.1 and 0.042 hPa, MLS diurnal changes are weaker than ground-based and model variations, but daytime O3 mixing ratios are found to be in very good agreement for all data sets. ¿ American Geophysical Union 1996